The present invention relates to a method and an apparatus for bending a glass sheet so that the glass sheet has a compound curvature for use in, e.g., an automobile window.
Herein, a glass sheet having a compound curvature is defined as a glass sheet of three-dimensional shape, especially curved with desired curvature radii along both X- and Y-axis directions and thereby being convex in a Z-axis direction according to the orthogonal coordinate system. A glass sheet curved simply in one direction to be part-cylindrical is referred to as a glass sheet having a simple curvature. Further, the terms “upstream” and “downstream” are defined in relation to a direction in which the glass sheet is conveyed on one production line.
There are several methods for bending a glass sheet such as an automotive window glass. In one method, a glass sheet is heated to become soft and bent by making the heated glass sheet sag by gravitation in a mold. In another method, a glass sheet is heated and then pressed by press molds. In still another method, a glass sheet is heated and bent while floatingly conveyed over hearth beds in a furnace. More specifically, while the glass sheet is conveyed over a series of hearth beds in a conveyance direction, hot gas is ejected toward the glass sheet so that the glass sheet is floated over the hearth beds. The hearth beds have upper surfaces curved along a direction perpendicular to the conveyance direction. The glass sheet is thus heated with the hot gas, gradually sags by gravitation and is bent according to the curvature of the upper surfaces of the hearth beds. In a subsequent stage, the bent glass sheet is quenched in a quenching unit. Alternatively, the glass sheet may be conveyed by a series of rollers in the furnace and bent according to the curvature of the rollers. A decision as to which method should be used is made depending on the size and curvature of bent glass sheets to be fabricated.
Among the above methods, the method using the hearth beds in the furnace (hereinafter referred to as a hearth bed bending method) has the advantage in its productivity for bending the glass sheet along the direction perpendicular to the conveyance direction. Thus, the glass sheets having a simple curvature can be fabricated at low cost by the heath bed bending method. In such a hearth bed bending method, it has been proposed to heat, bend and quench the glass sheets while conveying the glass sheets either upwardly or downwardly along the conveyance direction in order to also bend the glass sheets along the conveyance direction with a curvature radius of tens of thousand of millimeters. However, it is not easy to bend the glass sheet along the conveyance direction while conveying the heated glass sheets because of the furnace structure.
For example, U.S. Pat. No. 3,409,422 discloses an apparatus in which some hearth beds situated most downstream in a furnace and the lower air blower of a quenching unit are inclined downwardly along a conveyance direction to form a curvature curved in both of the conveyance direction and a direction perpendicular to the conveyance direction, when the apparatus is equipped to fabricate glass sheets having a compound curvature.
Further, U.S. Pat. No. 6,014,873 discloses an apparatus in which some hearth beds situated most downstream in a furnace are inclined upwardly along a conveyance direction so that the final hearth bed and the lower air blower of a quenching unit form an upwardly convex curvature, when fabricating glass sheets having a compound curvature.
U.S. Pat. No. 5,522,912 discloses an apparatus in which the final hearth bed has a top surface of which at least a downstream part is curved in a conveyance direction and is inclined upwardly, for fabricating glass sheets having a compound curvature.
In each of the above conventional apparatuses, the equipment thereof (especially, the final hearth bed) needs to be replaced and adjusted, when a different kind of product is to be fabricated. More specifically, in the U.S. Pat. Nos. 3,409,422 and 6,014,873, the above-mentioned downstream hearth beds and quenching unit need to be replaced with new ones. Then, the newly equipped hearth beds and quenching unit have to be positioned in place by adjusting their respective supports so as to be properly inclined and connected with each other. In U.S. Pat. No. 5,522,912, the final hearth bed has to be replaced with a new one. The newly equipped bed needs to be inclined upwardly by adjusting its support properly. Such replacement and adjustment require much time and labor, whereby the profitable merit of the hearth bed bending method (i.e., high productivity) cannot be maintained. In addition, there arises a problem that the position to take out the bent glass sheets from the apparatus may become too low, when the glass sheets are conveyed downwardly along the conveyance direction through the apparatus.